This invention relates generally to temporary and provisional dental crowns, and more particularly to flexible dimension crown shells and methods of making temporary and provisional dental crowns using such shells. Presently, there are three primary methods for the fabrication of temporary and provisional crowns. In the first technique, conventional prefabricated crown forms or shells, made of a metal such as aluminum or stainless steel, or of polycarbonate such as the Ion crown forms sold by 3M Corporation, are trimmed and shaped to fit a prepared tooth. Other examples of temporary crowns of this type are disclosed in U.S. Pat. Nos. 4,015,332 (Manne); 4,678,435 (Long); 4,778,386 (Spiry) and 5,458,489 (Tennyson).
The second technique calls for making an impression of the tooth before the tooth is prepared for a crown. Then the impression is filled with a bis-acryl material and placed over the prepared tooth. The material sets and is then removed from the dental impression, trimmed, polished and seated in the mouth.
The third primary technique, used and recommended by Gordon Christenson, is also popular. A putty-like ball of polymethyl-methacrylate is applied over a prepared tooth, the patient bites down, and the material begins to set. Before it completely sets, the putty-like material is removed from the tooth, trimmed and placed back on the tooth. The set material is then trimmed and the bite is adjusted, and the temporary crown is cemented to the tooth. A variation in this technique, disclosed in U.S. Pat. No. 5,385,469, uses a tubular dental form for forming a universal crown in situ.
Each of these techniques has various advantages and disadvantages. The first technique, using prefabricated forms, is fast and simple, but the fit of the conventional shell is not very good. The margins do not fit well. It is hard to get good proximal contact to adjacent teeth, and the contours and occlusion are not always good. Some manufacturers try to overcome these drawbacks by proliferating sizes and shapes of the shells. This approach is expensive, and turns out to be very time-consuming for the dentist to pick the right size and shape out of a selection of as many as 80 different sizes and shapes of molars and bicuspids. Manne adds a degree of freedom by providing an incisor shell that has slits in the mesio-distal sides to permit the shell to flex in the labio-lingual direction about a hinge axis at the occlusal surface. Long provides a degree of freedom in a temporary molar crown by having the mesio-distal sides open to permit the acrylic resin filler material to protrude proximally to contact adjacent teeth. These shells, like others used in this technique, require trimming the free edges of the buccal and lingual sidewalls to assure a good fit along the gingival margins as well as a good occlusion. The Long shells, with open mesio-distal sides, also appear to require substantial trimming of the filler material. Such trimming and fitting is time-consuming for the dentist and patient.
The second technique gives good contours and bite accuracy but it is time-consuming and the strength and durability of the temporary crowns are not very good. Making impressions is time-consuming, and cannot be done if the patient's tooth is already broken when initially treated. The third technique of free-forming a temporary of putty-like material is accurate and fairly fast if performed by a skilled dentist or technician. The main problem with this approach is that it is very technique-sensitive. A dental technician must be highly skilled in order to carve accurately the tooth anatomy. Another problem is that special care must be taken to assure that the patient's mouth is not injured by the exothermal reaction that is involved in the curing of the putty-like material.
All of the foregoing techniques take a half an hour or more work for the dentist to fit a temporary or provisional crown to a patient. The resulting temporary or provisional crown is typically not very durable and therefore not well-suited for long-term wear. An exception is stainless steel shells which are very durable but these are more difficult to fit and to grind the occlusal surfaces to get a good comfortable bite.
Accordingly, a need remains for a way to make temporary and provisional crowns that is quick and accurate, provides a good fit without much trimming, is durable enough for long-term use, and is inexpensive.